It has been known that the electrostatic build-up of charge on insulating surfaces in a high vacuum arrangement which operates with a charge carrying or transporting beam, may considerably influence the function of such high vacuum apparatus, inasmuch as the electrostatic stray fields produced by the electrostatic charge build-up may cause frequently an undesirable deflection of the charged particles.
A local accumulation of the interfering electrostatic outer surface charge build-up on the upper surface of a structural member in a high vacuum arrangement, which surface is exposed to the vacuum, may appear not only when the structural member is made from an electrically insulating material, such as glass or ceramic, but also in the cases when the structural member is made from metal on which, during the manufacturing or operation, an insulating upper or free surface layer has been formed. Especially the oxides of many metals which are being used as structural members in high vacuum arrangements, such as aluminum, copper, tantalum, chromium and iron metals, form oxide layers which, already at a very thin layer thickness, can bring about a good insulation and can lead, at the same time, to disturbing charge accumulations on the upper surface of the structural members, especially when relatively many charge carriers per time unit fall on the upper surface.
Also in many other apparatus and arrangements, problems might appear due to the fact that a metal upper surface is exposed during operation of the apparatus to the atmosphere or to a gas having an atmospheric pressure or above it, and, as a result, by oxidation or other type of changes it becomes electrically insulating or at least becomes a relatively bad conductor. This is more so when the upper surface during operation will go into an alternating relationship with an electric field and/or when a local charge build-up on the upper or free surface will, or may cause disturbances. Illustrative examples to this point are the electrodes of rotating condensers, radiators and antennas for electromagnetic waves, especially for ultrashort and microwaves, such as horn radiators and parabolic mirror antennas, wave guides, cavity resonators, as well as the contact surfaces and upper surfaces of the stators and rotors of a centrifuge, especially those of an ultra centrifuge, or those of a turbo-molecular pump, as well as the surfaces which in operation move at a high speed with respect to each other, which might be for example, exceed 10 m/s.
It became known from West German OS No. 2639033, having its U.S. counterpart as U.S. Pat. No. 4,123,655, to produce structural elements for high vacuum apparatus from metals having low spattering rate and a very small desorption rate, such as from titanium, zirconium and similar materials and their alloys; however, nothing is mentioned about the electrical character of the upper surfaces involved. The same is true about West German OS No. 2500339, which discloses a particle trap for molecular, atomic or subatomic particles, which comprises a three-dimensional network which forms a plurality of mutually interconnected free lines and which may be made from graphite, copper, nickel, chromium, iron, titanium, wolfram, cobalt, molybdenum and similar materials. In such known particle trap, however, there is no weight or consideration given to produce an upper or free surface which is free of charge build-up or that a possible and undesirable build-up of charge on the upper surface may make the collection by the charge carriers difficult or prevent it completely. On the contrary, structural elements can be made which are similarly shaped as the one proposed by the present invention, especially structural elements having a plurality of narrow holes. This can be seen in the West German OS No. 2,639,033, however, without any special treatment of the upper surfaces, even if such structural elements are being proposed as time durable, stable and insensitive particle traps for charged particles.